Monolithic converters have in the prior art used expandable mica mat pieces to support a ceramic substrate in a metal housing. Some of such substrates have been oval in cross section and others circular. In each it is desirable to control the mount density of the mat where it supports the substrate. Intumescent mats for this purpose are selected from an inventory of mat pieces having different predetermined thicknesses. The mat thickness selected is chosen in accordance with the expected nominal gap between the ceramic substrate and its metal housing. A tongue and groove configuration is used at opposite ends of such mat pieces to interlock the mat ends when the mat is wrapped around the substrate. But the tolerance requirements of the substrate's peripheral dimension needs to be precise if the mat ends are to properly interlock and therefore the substrate is costly to produce.
In such prior art converters the tongue and groove relationship at the ends of a mat piece wrapped around a substrate may become out of alignment. Such out-of-alignment may be caused by variations in the peripheral dimension or perimeter of the substrate about which the mat piece is wrapped. If the substrate dimension strays from the nominal size on the large side, the tongue and groove ends of the mat piece are not closely juxtaposed. This creates the possibility that automobile exhaust gas can bypass the catalyst in the converter by flowing between the spaced-apart ends of the mat piece. Such flow increases the potential for erosion of the mat along the ends of the mat.
On the other hand if the substrate dimension strays from the nominal size on the small side, the tongue and groove ends of the mat piece may overlap each other. This creates the possibility that the double thickness of mat will bulge the converter housing thereat during intumescent expansion of the mat, again raising the possibility of exhaust gas bypass around the catalyst and erosion of the mat at the bulge. The bulge may also apply sufficient pressure to the substrate to fracture the substrate.
In some prior art circular monolith converters with variably sized substrates, the density of the mat is controlled by the cylindrical metal housing. This is done by squeezing or collapsing the overlapping ends of a rectangular sheet metal housing to tightly surround or hug the mat and substrate. The squeezing continues until the resultant external peripheral dimension of the housing gives the desired mat density. Then the housing ends are welded together. See, for example, U.S. Pat. No. 4,070,158.
U.S. Pat. No. 3,959,865 recognizes that it would be advantageous not to fabricate ceramic substrates to close tolerances for mounting in a housing also held to a rather close tolerance. The teaching of this patent, however, is to use organic or inorganic foam which is formed in situ between substrate and housing at an elevated temperature.
U.S. Pat. No. 4,048,363 discloses laminated intumescent mat supplied in roll form and cut to lengths which correspond to the periphery of the substrate. A single thickness of the expandable laminate is used for each substrate irrespective of the gap size between the substrate and its housing.
U.S. Pat. No. 3,861,881 also seeks to eliminate the need for stringent dimensional tolerances. To do so the patent discloses a fibrous ring made up of spirally wound layers which can be paper thin and sufficient in number to build up the thickness in accordance with the degree of thermal and shock insulation desired. The ring is not intumescent and is not tailored to a specific substrate. Thus, when the ring is press fitted over the substrate, the resultant peripheral dimension will vary depending on the substrate.
U.S. Pat. No. 4,239,733 discloses a catalytic converter having two serially arranged catalyst coated monoliths of frangible ceramic material supported in a sheet metal housing at least partially by an intumescent sleeve.